docs: def _find_boundary_vars(self, ds, refresh=False):
          '''
          Returns dictionary of boundary variables mapping the variable instance
          to the name of the variable acting as a boundary variable.
  
          :param netCDF4.Dataset ds: An open netCDF dataset
          :param bool refresh: if refresh is set to True, the cache is
                               invalidated.
          :rtype: list
          :return: A list containing strings with boundary variable names.
          '''
          if self._boundary_vars.get(ds, None) and refresh is False:
              return self._boundary_vars[ds]
  
          self._boundary_vars[ds] = cfutil.get_cell_boundary_variables(ds)
  
          return self._boundary_vars[ds]